(1) Field of the Invention
The present invention relates to a phase shift mask for manufacturing a semiconductor device, and in particular to a phase shift mask to be applied to an imaging apparatus and the like, and a method for manufacturing a light-collecting device using the phase shift mask.
(2) Description of the Related Art
In general, apparatuses which convert images into electric signals (such apparatuses are referred to as imaging apparatuses) are used in appliances which electromagnetically record images of such as digital video recorders, digital still cameras, and camera-equipped cellular phones which have been rapidly growing in number. In recent years, a charge-coupled device sensor which is a type of a semiconductor device (this is commonly referred to and abbreviated as a CCD sensor, hereinafter) and a MOS sensor are used as such imaging apparatuses. The introduction of such sensors has contributed to make the appliances smaller and lower-priced. Each of such imaging apparatuses is made up of fine-pixels respectively including one photodiode that are arranged on a plane. Accordingly, performance of an imaging apparatus is determined depending on the performances of these pixels.
The particularly significant performances of the imaging apparatus are a capability of converting a fine input image into an electric signal with low noise (i.e. low S/N ratio) and a capability of outputting the input image as a high electric signal (i.e. with high amplification factor).
As a method for realizing such a low S/N ratio and high amplification factor, a method for improving the S/N ratio and amplification factor of an imaging device in a pixel is generally suggested. Here, the following method is also commonly adopted.
FIG. 1 is a cross-sectional view of an imaging device (pixel unit) in a typical conventional imaging apparatus. A pixel 801 includes an imaging device 802, a light-collecting device 803, and a color filter 804. The incident light 805 entering the pixel 801 is collected by the light-collecting device 803, separated into one of red, blue, and green light by the color filter 804, and inputted into the imaging device 802. The intensity and density of the incident light 805 entering the imaging device 802 is increased by the light-collecting device 803. Therefore, the improvement in a low S/N ratio and an amplification factor can be realized.
Here, a problem is that a focal point of the light-collecting device 803 is changed along with the change of the incident angle of the incident light 805, so that the light can not be collected on the imaging device 802. This is obvious in the case where the pixel 801 is a peripheral pixel in the imaging apparatus.
In order to overcome this problem, there is a conventional example of arranging light-collecting devices in each pixel so as to be asymmetrical to each other (e.g. Japanese Unexamined Patent Application Publication No. 2001-196568). In addition, in a peripheral pixel of the imaging apparatus, the position of the imaging device 802 with respect to the light-collecting device 803 is conventionally displaced. However, in such a conventional example, when the incident angle for the incident light 805 is relatively small, an effect of displacing the position is high. However, a problem is that the effect becomes low when light enters at a large incident angle.
Here, in order to maintain pixel characteristics even in the case where light enters at a large incident angle, Japanese Unexamined Patent Application Publication No. 2004-117689 discloses a technique of forming a light-collecting device as shown in FIGS. 2A and 2B. 901 is a light-transmitting film, 902 is a substrate, 903 is a color filter, 904 is an imaging device, and 905 is the incident light. The light-transmitting films 901 are formed into circles sharing the same center or into zones. The width of a zone is as long as a wavelength of natural light. The width of a zone is typically as long as 0.1 μm. The refractive index for the incident light 905 which passes through the light-transmitting films 901 is a value averaged by a region as large as a wavelength on the surface of the light-transmitting films 901. This value is neither a value of the refractive index of the light-transmitting films 901 nor a value of the refractive index of a medium (typically, air). Since the width of a zone is very small, the refractive index of the incident light 905 depends on the width of the zone, and becomes a value between the refractive index of the light-transmitting films 901 and the refractive index of the medium. Specifically, the incident light 905 enters the surfaces of the light transmitting films 901 in which refractive indexes are distributed in concentric circles. Due to this refractive index distribution, the incident light 905 which has passed through the light-transmitting films 901 and the substrate 902 is collected by a diffraction effect and reaches the imaging device 902. A position where the incident light 905 is collected can be controlled by changing a shape of the light-transmitting films 901. Accordingly, incident light can be collected on the imaging device 902 without causing performance deterioration by designing the shape of the light-transmitting films 901 in consideration with an incident angle of the incident light 905, so that the aforementioned object can be achieved.
Actually, the center of the light-collecting device shown in FIGS. 2A and 2B is repeatedly formed as shown in FIG. 3. 1001 is a light-transmitting film, 1002 is a pixel boundary, and 1003 is an aperture.
In order to manufacture a light collecting device with such a structure using a photolithography technique, it is necessary to provide a photomask having a light-blocking portion in a region corresponding to the light-transmitting film 1001 and having a light-transmitting portion in a region corresponding to the aperture 1003 (although there may be the opposite case depending on a manufacturing method, it is assumed that the light-blocking portion is placed in a region corresponding to the light-transmitting film 1001, hereinafter).
When manufacturing the light collecting device, a problem is that it is difficult to fine-process circles or zones as fine as less than half of a wavelength of natural light for image-taking (approximately 100 nm). However, there is a problem when a conventional photomask is used. In other words, when a structure having a pattern less than a wavelength of a light source which is used for photo lithography is manufactured, large variations in a resist dimension occur due to the exposure variations in planarization on a substrate to be exposed to light. Alternatively, there are cases where a desired structure does not appear as a resist pattern. Thus, it is difficult to manufacture the desired fine structure precisely.
The current exposure source is light using KrF (wavelength of 0.248 μm) or light using ArF (wavelength of 0.193 μm). This indicates that it is possible to manufacture only a structure at a wavelength of approximately 0.2 μm.
For solving this problem, it is conceivable that a phase shift mask is used as a photomask for exposure. There are two types of phase shift masks, a halftone type and an interleave (Levenson) type. It is known that the Levenson type is more effective for the fine processing. The Levenson type phase shift mask (simply referred to as phase shift mask, hereinafter) is characterized in that light-transmitting portions and phase shift portions are alternately arranged having a light-blocking portion in between. Exposure light which passes through a phase shift portion is shifted by 180° in phase with respect to exposure light which passes through a light-transmitting portion.
In the phase shift mask, the following principle enables the fine processing. First, conventionally, constituent elements of the mask include only light-blocking portions and light-transmitting portions. Thus, when the width of the light-blocking portion is small, light is diffracted underneath the light-blocking portion due to a diffraction phenomenon, and a resist underneath the light-blocking portion is exposed to light. Thus, it becomes difficult to realize the fine structure. On the other hand, in the phase shift mask, light-transmitting portions and phase shift portions are arranged having light-blocking portions in between. Thus, the light which is diffracted underneath the light-blocking portion includes light from a light-transmitting portion and light from a phase shift portion. In this case, since the phases of each light are opposite to each other, the amplitude becomes small after the light is combined, and the exposure intensity is suppressed, so that the fine processing can be achieved even in the fine structure. Furthermore, an effect of improving contrast of the exposure intensity and the focal depth can be produced.
When this phase shift mask is applied for manufacturing the light-collecting device shown in FIGS. 2A and 2B, the following problem occurs. When the light-transmitting film 1001 which is the innermost located among the light-transmitting films 1001 of the light-collecting device is manufactured using the phase shift mask, a portion which is adjacent to the light-blocking portion on the phase shift mask corresponding to the innermost located light-transmitting film 1001 is either the light-transmitting portion or the phase shift portion. Thus, in these portions, it is difficult to realize the fine structure. The reason is that according to the principle of the phase shift mask, a light-transmitting portion and a phase shift portion appear, only when they are adjacent to each other having a light-blocking portion therebetween.
In other words, when a concentrically arranged pattern is manufactured, there is a problem that the effect of the phase shift mask can not be produced in some portions and it is difficult to perform the fine processing.
The object of the present invention is to provide a phase shift mask that is concentrically arranged and a method for manufacturing a light-collecting device which enhances the precision of the fine structure of a semiconductor apparatus.